XML and Its Applications Ben Y. Zhao, CS294-7 Spring 1999

Overview: The XML Language n What is XML n Document Type Definitions n XML and DTD Example n XML APIs: DOM and SAX n Pros and Cons

What is XML? n Extensible Markup Language is a simplified subset of Standard Generalized Markup Language. n Tags can be arbitrarily named, and can be used to encode semantic information about enclosed data. n Documents can be well-formed, or optionally validated against Document Type Definitions. n XML documents can be componentized, and be distributed across networks. n Creates a self-describing, text-based framework around text and binary data

Document Type Definitions n DTD: Concise structural definition of an XML document type. n DTDs are optional, well-formed+DTD=>valid n DTDs provide a way to enforce XML documents’ compliance to constraints on XML documents. n Optional elements and attributes add additional flexibility to support evolving schemas. n DTDs can be embedded in the XML document, providing a self-sufficient validating document.

XML and DTD Example (open editor) Ben Zhao

XML APIs: DOM and SAX n DOM (Document Object Model) – Provides a definitive API for accessing hierarchical description languages like XML, HTML – Specifies interfaces for accessing all part of a document – Includes inheritance, typing, and constants n SAX (Simple API for XML) – An event-driven parser API – API reports parsing events to application via callbacks – Optimized for parsing large documents by eliminating need to keep tree structure in memory

Pros and Cons Pros n Simple (human readable) n Standard (easy to integrate, widely adopted) n Portable (cross-platform data exchange) n Flexible (handles complex data) n Extensible (dynamic data model) Cons n Text-based means space consuming n Standardization is still a problem to be solved n Evolutionary model means ill-defined functionality core

Overview: Current XML Efforts n XML Tools n Evolving Recommendations n Industry Databases n XML Query Languages n Research Query Engines n Relevance to Systems Research

XML Tools n Parsers – Existing parsers support DOM or SAX – Varying XML compliance and performance n Editors for XSL, XML and DTDs n Browsers n Converters – Applications that convert from and to XML n Document Management – Lightweight searching and indexing tools – Difference engines

Related Evolving Recommendations n Namespaces: qualifying names with URI references n XML-data, defines XML vocabulary for schemas (definitions of characteristics of classes of objects) n XLink – XML Linking language, sophisticated link styles n XPointers – XML Pointers to all parts of XML documents n RDF (Resource Description Framework) – model for using XML to describe metadata on the web n DCD(Document Content Description) – XML-data + RDF

Industry XML Databases n ObjectStore eXcelon – Middle tier server that imports from different DB stores – XQL queries applied to integrated data – Provides “cache server” for XML imported from heterogeneous DB backends – Focuses on web applications as access methods to DBs n Poet XML Repository – Object oriented database with standard DB functionality, with OQL – Focuses on use of XML to faciliate EDI

XML Query Languages n XML-QL (AT&T, Inria, U.Wash.) – Very similar to SQL – Optimizations and other DB techniques applicable – Data integration and conversion from hetero. sources n XQL (Microsoft) – Based on the XSL transformation language – Context based and XML-specific query matching – Departure from the database-centric SQL format n LOREL (Stanford) – See notes from Last Week’s LOREL presentation

Research Query Engines n LORE (Stanford) – Based on the LOREL query language – A feature-rich DB approach to XML storage and query, with context-free indexing, path indexing through dataguides, query optimizations, and views n XSet (UCB, Ninja) – Streamlined XML search engine implemented in Java – Focus on high performance rather than feature set – Small size favors integration into low-level applications – Research issues on next slide

Relevance to Systems Work n FSML: XML meta-index for fast access to files n Distributed Service Discovery (Ninja SDS) – Service descriptions encoded in XML n Semantically Enhanced Web searching n Data exchange across heterogeneous platforms n Low overhead scripting language for thin clients n User preferences – Embedded logic and scripting inside XML

Discussion n XML: flexible description language with optional DTD validation n Provides flexible framework for marking data with inferred semantics n Provides additional push towards standardization, but not as a result of the language itself n Are the benefits of the XML movement due to something intrinsic in the language? n Description language of choice? Pervasive among future applications?